Controlling static charge is an important issue in semiconductor manufacturing because of its significant impact on the device yields. Device defects caused by electrostatically attracted foreign matter and electrostatic discharge events contribute greatly to overall manufacturing losses.
Many of the processes for producing integrated circuits use non-conductive materials which generate large static charges and complimentary voltage on wafers and devices.
Air ionization is the most effective method of eliminating static charges on non-conductive materials and isolated conductors. Air ionizers generate large quantities of positive and negative ions in the surrounding atmosphere which serve as mobile carriers of charge in the air. As ions flow through the air, they are attracted to oppositely charged particles and surfaces. Neutralization of electrostatically charged surfaces can be rapidly achieved through the process.
Air ionization may be performed using electrical ionizers which generate ions in a process known as corona discharge. Electrical ionizers generate air ions through this process by intensifying an electric field around a sharp point until it overcomes the dielectric strength of the surrounding air. Negative corona occurs when electrons are flowing from the electrode into the surrounding air. Positive corona occurs as a result of the flow of electrons from the air molecules into the electrode.
To achieve the maximum possible reduction in istatic charges from an ionizer of a given output, the ionizer must produce equal amounts of positive and negative ions. That is, the output of the ionizer must be "balanced." If the ionizer is out of balance, the isolated conductor and insulators can become charged such that the ionizer creates more problems than it solves. Ionizers may become imbalanced due to power supply drift, power supply failure of one polarity, contamination of electrodes, degradation of electrodes, or ambient air conditions such as changes in permeability or humidity. In addition, the output of an ionizer may be balanced, but may drop below its desired level due to system component degradation.
Accordingly, ionization systems incorporate monitoring, automatic balancing via feedback systems, and alarms for detecting uncorrected imbalances and out-of-range outputs. Most feedback systems are entirely or primarily hardware-based. Many of these feedback systems cannot provide very fine balance control, since feedback control signals are fixed based upon hardware component values. Furthermore, the overall range of balance control of such hardware-based feedback systems may be limited based upon the hardware component values. Also, many of the hardware-based feedback systems cannot be easily modified since the individual components are dependent upon each other for proper operation.
A charged plate monitor is typically used to calibrate and periodically measure the actual balance of an electrical ionizer. The charged plate monitor is also used to measure static charge decay time. If the decay time is too slow or too fast, the ion output may be adjusted by increasing or decreasing the preset ion current value. This adjustment is typically performed by adjusting two trim potentiometers (one for positive ion generation and one for negative ion generation).
Ionization systems may be used to control static charge in an entire room or in a predefined work surface area. FIG. 1 shows a conventional "overhead ionized air blower" or "overhead ionizer" 10 for controlling static charge on top of work surface 12. The overhead ionizer 10 provides a cushion of ionized air protection above the work surface 12, such as from 0-4 inches above the work surface 12. The overhead ionizer 10 is typically hung over the work surface, such as about 30 inches above the work surface. The overhead ionizer 10 includes therein a plurality of ionizers 14 and a plurality of fans 16, each fan being associated with one ionizer 14. One conventional scheme uses three pairs of ionizers 14.sub.1 -14.sub.3 /fans 16.sub.1 -16.sub.3. The fans 16 create a unidirectional airflow downward from the ionizer 10 to the work surface 12. Power is provided to the fans 16 in parallel with the respective ionizers 14 so that both are either on or off. Fan speed can be adjusted, but the adjustment simultaneously adjusts all fans equally.
When a reading from a charged plate monitor detects an ion imbalance or insufficient ion output, the ion balance and/or ion output must be adjusted. Conventional ionizers 14 contain analog trim potentiometers or digital potentiometers for making such adjustments. To make adjustments in a conventional overhead ionizer 10, a person must reach up to the overhead ionizer 10 to adjust the analog potentiometers or to press UP/DOWN buttons which control digital potentiometer settings. Each ionizer 14.sub.1 -14.sub.3 has a separate set of potentiometers. One significant problem with the conventional balance adjustment scheme is that the person's physical movements for performing the adjustment and the physical presence of the person in or near the cushion of ionized air protection interferes with proper adjustment and may introduce sudden, large static charges into the work area.
Automatic balance control systems in conventional ionizers are also inherently limited in how quickly and precisely they can achieve a balanced condition. In a conventional automatic balance control system, imbalances are detected by iteratively measuring balance over a plurality of past time periods and then guessing how to adjust one or both of the positive and negative power supplies to achieve a balanced condition in subsequent time periods. This scheme has at least two significant problems. First, by sampling past time periods to determine subsequent adjustments, the scheme introduces a lag time in the balance adjustments during which time the ionizer is imbalanced. Second, this scheme cannot provide a long-term balanced condition. That is, if the ionizer is too positive for a few milliseconds, the ionizer merely corrects for the excess positive ions by moving towards a balanced condition wherein there are a lesser amount of excess positive ions. No effort is made to compensate for the few milliseconds of being too positive, such as by being too negative by the same amount for a few milliseconds. These two problems limit the ability of conventional balance schemes to provide ideal short-term and long-term balanced conditions.
Accordingly, there is an unmet need for a scheme which allows overhead ionizers to be adjusted without interfering with the static field in the work area to be neutralized. There is also an unmet need for an improved balance adjustment scheme. Furthermore, there is an unmet need to allow ionizer fans to be operated in a more flexible manner with respect to their ionizer. Lastly, there is an unmet need for a fast and precise balance adjustment scheme. The present invention fulfills these needs.